The present invention relates to oceanographic phenomena, more particularly to the relation underwater between electromagnetic fields and ocean water dynamics.
Understanding the characteristics of naturally occurring underwater electromagnetic fields and their relationship to physical oceanographic properties within a coastal environment is important to the study of oceanography and related engineering disciplines. Ocean dynamics appear as variable magnetic anomalies that change on the scale of the ocean features they denote.
The motion of the electrically conductive water through the earth's magnetic field causes an important source of extremely low frequency electromagnetic (EM) variations in the ocean. Movement of seawater in the earth's magnetic field produces an electromotive force with an associated electric current and magnetic field. As a result, surface waves, internal waves, solitary waves, tides, and ocean currents all produce observable magnetic and electric fields.
These kinematic ocean features contribute to the magnetic field that magnetic sensors observe when measuring the field in or over ocean areas. As a result, the ocean dynamics appear as variable magnetic anomalies that change on the scale of the ocean features they represent. For stationary magnetic sensors like those deployed on the bottom of the seafloor, the ocean dynamics should appear as periods of increased magnetic background noise. For airborne magnetic sensors traveling across a segment of ocean, the ocean dynamics should appear as non-stationary anomalies that move or change with time.
Early studies of magnetic and electric fields generated by ocean flow were concerned with electric fields induced by the steady motion of seawater. Internal waves have been observed with magnetic sensors in the deep ocean and are routinely characterized by measuring the electric and thermal structure using in-water sensors.
Theoretical models for internal wave induced magnetic spectra indicate that the amplitude increases with decreasing frequency, and predictions of the influence on magnetic surveys have been calculated based on these models. A model was developed for the induced magnetic field from internal waves in a two-layered ocean. A more comprehensive treatment of internal waves followed for an exponentially stratified ocean with a horizontally uniform Brunt-Vaisala frequency profile. Subsequently a spectral estimate was generated of the magnetic induction. A somewhat more general solution was derived for internal waves that also used the wave spectra and presented predictions of the magnetic power spectra above and below the water surface.
Further understanding is desired of the extent to which direct measurement of ocean dynamics can reduce electromagnetic sensor noise. In particular, it would be beneficial to be capable of quantifying oceanographic dynamic influence on electromagnetic fields so as to compensate the resultant noise in electromagnetic field measurement, especially in coastal regions.
The following reference is pertinent to the instant disclosure and is incorporated herein by reference: W. E. Avera, Patrick C. Gallacher, and W. J. Teague, “Magnetic Noise Associated with Ocean Internal Waves,” IEEE, Oceans, 2009, 26-29 Oct. 2009. See also the following references that are pertinent to the instant disclosure: M. S. Longuet-Higgins, M. E. Stern, and H. Stommel, “The Electric Field Induced by Ocean Currents and Waves, with Applications to the Method of Towed Electrodes,” Papers in Physical Oceanography and Meteorology XIII, I, Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1954; H. T. Beal and J. T. Weaver. “Calculations of Magnetic Variations Induced by Internal Ocean Waves,” J. Geophys. Res., vol. 75, no. 33, 1970; W. Podney, “Electromagnetic Fields Generated by Ocean Waves,” J. Geophys. Res., vol. 80, no. 21, 1975; R. A. Petersen and K. A. Poehls, “Model Spectrum of Magnetic Induction Caused by Ambient Internal Waves,” J. Geophys. Res., vol. 87, no. C I, pp 433-440, 1982; C. Garrett and W. Munk, “Space-time Scales of Internal Waves,”J. Geophys. Fluid Dynamics, vol. 2, pp. 225-264, 1972; A. D. Chave, “On the Electromagnetic Field Induced by Ocean Internal Waves,” J. Geophys. Res., vol. 89, no. C6, pp 10519-10528, 1984; W. E. Avera, “Influence of Internal Wave Ocean Dynamics on Magnetic Surveys,” MARELEC 2009 Conference, Stockholm Sweden, Jul. 7-9, 2009; W. A. Venezia, et al., “Successful Navy and Academic Partnership Providing Sustained Ocean Observation Capabilities in the Florida Straits,” Marine Technology Society Journal, vol. 37, no. 3, pp 81-91, Fall 2003; W. A. Venezia, “Buoy Systems to Augment a Narrow Continental Shelf Cabled Ocean Observatory,” in ONR/MTS Buoy Workshop, Monterey, Calif., March 2010; C. Bradley and W. A. Venezia, “Spar Buoy Platform for Water Wave, Turbulence and Underwater Electric Field Sensors,” Proceedings of the IEEE/OES/CWTM Tenth Working Conference on Current Measurement Technology, April 2011; M. Dhanak, W. Venezia, E. An, R. Couson, J. Frankenfield, and K. von Ellenrieder, “Magnetic Field Surveys of Coastal Waters Using an AUV-towed Magnetometer,” Oceans, 2013; A. Soloviev, M. Silvia, W. Avera, “Analysis of the Electromagnetic Signatures of Fine-Scale Oceanographic Features,” MARELEC, 2013, Hamburg, Germany, 16-19 Jul. 2013; W. Avera, J. Bradley Nelson, and W. J. Teague, “Comparison of In-Water Predicted and Measured Magnetic Fields Due to Ocean Dynamics,” MARELEC 2011, San Diego Calif., 20-23 Jun. 2011; J. Bradley Nelson and W. Avera, “Comparison of Ocean-Generated Magnetic Fields Measurements in Air and Water versus Predicted from Ocean Circulation Models,” MARELEC 2013, Hamburg, Germany, 16-19 Jul. 2013; J. N. Moum, and W. D. Smyth, “The Pressure Disturbance of a Nonlinear Internal Wave Train,” J. Fluid Mech. 2006, vol. 558, pp 153177; A. R. Osborne, Nonlinear Ocean Waves and the Inverse Scattering Transform, 1st ed. Burlington: Academic Press, 2010; J. Clerk Maxwell, A Treatise on Electricity and Magnetism, 3rd ed., vol. 2, Oxford: Clarendon, 1892, pp 68-73.